Abstract: The spiral tube of a heater leaked in operation. The cause of the leakage was analyzed by macromorphology and microstructure observation, chemical composition analysis, scanning electron microscopy, energy disperse spectroscopy, and so on. The results show that creep cracking occurred in the spiral tube, and the cracking form was brittle intergranular cracking. Because of the low working temperature, flake carbides precipitated on grain boundaries of the welding heat-affected zone of the spiral tube, causing the formation of chromium-depleted zones nearby. Under the bending stress, vibration stress and welding residual stress, many cavitation defects formed around the carbides, and the cavitation expanded and grew, forming cracks and finally leading to leakage.